Multiple Endocrine Neoplasias are characterized by a series of oncogenic events. Patients with the most severe form, MEN2B, exhibit metastatic medullary thyroid carcinoma (MTC), pheochromocytomas, ganglioneuromas, and mucosal neuromas, in addition to earlier developmental defects. Most patients diagnosed with MEN2B contain a dominantly-acting point mutation in the intracellular tyrosine kinase catalytic domain of the Ret receptor tyrosine kinase, resulting in ligand-and dimerization-independent activation of the receptor. Similar effects have been observed with sporadic MTCs, which often have the same M918T mutation. The specific effect of this mutation is not known, but biochemical evidence suggests that the catalytic specificity of the receptor is altered. The identity of this putative new signaling pathway is not known. This proposal seeks to use the Drosophila retina as a model system to identify both the signaling pathways activated by the M918T mutation and to identify candidate drug targets. The Drosophila retina was chosen due to the availability of powerful molecular and genetic tools and the exceptional accessibility of the retina to genetic screens. The orthologous mutation was made in the Drosophila Ret ortholog dRet; the mutant construct was fused to a retinal-specific promoter and stable transgenic lines were established. Expression of dRetM1007T in the retina resulted in overproliferation and developmental defects that are strikingly reminiscent of the symptoms observed in MEN2B and sporadic MTC patients. A dRetM1007T transgenic line was used in a genetic screen to identify genetic modifiers of the dRetM1007T phenotype. Preliminary data are suggestive of activation of at least two separable signal transduction pathways, at least one of which fails to demonstrate genetic interaction with an activated form of the dEGFR. Of particular importance was the identification of genetic suppressors, loci which when their genetic dose was halved brought the severe dRetM1007T phenotype nearly back to wild type. The corresponding genes are of particular interest as potential drug targets, as partial reduction of their activity may have important therapeutic effects. This Proposal seeks to further characterize the role of these genetic modifiers both in the Drosophila retina model system and directly in human medullary thyroid carcinoma tissue.